1. Field of the Invention
This invention relates to a multielement-sensor measuring device and more particularly to an improvement in a device for compensating for variations in offset and gain among the individual elements in a multielement-sensor measuring device.
2. Description of the Prior Art
In the case of a line sensor array for light detection, for example, a circuit for compensating for the variation in gain (variation in sensitivity) and the variation in offset (variation in dark current) is required in order to measure minute amounts of light with good precision. For this type of circuit, there is usually used one in which amplifiers connected to the individual sensor elements are provided with variable resistors by which adjustment is made independently for each sensor element. In this specification, the term "individual element" will be used in some cases to mean a single unit element and in others to mean a circuit consisting of a single unit element and an amplifier connected thereto. In the circuit referred to above, the adjustment in gain is made to compensate for variation in the sensitivity and quantity of light among the individual elements and the adjustment for offset is made to compensate for variation in dark current among the individual elements and the offset of the amplifier itself. Depending on what is required of the measuring device, the output of the amplifiers is passed to the next stage of signal processing either via a multiplexer (switching means) or directly.
In the system just described it is necessary to provide two variable resistors for each individual sensor, one for gain adjustment and one for offset adjustment. This has the following disadvantages: (1) The cost is high; (2) The physical size of the circuit is large; and (3) Adjustment is troublesome.
In another system, the outputs from the individual elements are fed first to a multiplexer and from the multiplexer to an amplifier. Although this system is free from the disadvantages mentioned above, it has the following defects: (1) High-precision measurement is impossible since no compensation is made for variation in sensitivity and dark current among the individual elements; and (2) The measurement of small currents is difficult since the leak current etc. of the multiplexer has a direct effect on the sensor output.